1. Field of the Invention
This invention relates to improved electrodes for rechargeable cells or batteries, their manufacture and use in cells containing same. More particularly, this invention relates to electrodes comprising a foraminous, conductive substrate containing a coating of electrochemically active material which has been coated with a latex material, their manufacture and use in rechargeable cells containing same.
2. Background of the Disclosure
The general manufacture and use of electrodes for electrochemical cells such as rechargeable cells is known to those skilled in the art. Such electrodes usually comprise a foraminous or perforated conductive metal substrate on which is deposited a layer of electrochemically active material. The electrochemically active material is usually applied to the substrate in the form of a paste which contains an organic binder material to provide mechanical integrity. The paste coated substrate is then subject to mild heat and pressure in order to form a compact layer of active material and dry the water or other liquids present in the paste.
Most rechargeable electrochemical cells in use today employ, as a negative electrode, a mixture of particles of cadmium oxide and cadmium metal which has been mixed with a binder material and applied to a substrate, preferably a foraminous, conductive substrate. Thus, U.S. Pat. No. 3,870,562 relates to a rechargeable cell containing a cadmium pressed negative electrode which comprises a mixture of cadmium metal spherical particles formed by condensing cadmium metal vapors in the presence of oxygen and having an average particle size of about 3-12 microns, cadmium oxide or cadmium hydroxide and a minor quantity of nickel hydroxide powder. In making the electrode, the powder mixture is blended with water and an aqueous dispersion of fluorocarbon to form a paste which is spread on the substrate, the coated substrate is then heated to a temperature not exceeding 200.degree. F. to break the fluorocarbon dispersion without sintering the fluorocarbon.
U.S. Pat. No. 3,877, 986 claims a cadmium negative electrode similar to that employed in the '562 patent. U.S. 3,888,695 claims an improved rechargeable cell wherein the negative electrode is a cadmium electrode which comprises a mixture of spherical cadmium particles having a particles size of about 3-12 microns and cadmium oxide powder which is combined with a binder and applied to a foraminous substrate. Preferably, the cadmium metal particles are stabilized with cadmium oxide. U.S. Pat. No. 3,954,501 claims a rechargeable electrolytic cell comprising positive and negative electrodes separated by a separator wherein the cell contains an electrolyte and wherein one of the cells comprises a foraminous electrically conductive substrate which supports a paste comprising finally divided, particulate, rechargeable, active material and a fibrous interconnecting network of an unsintered, uncoalesced, hydrophobic linear fluorocarbon polymer. In this electrode the active material is dispersed throughout and bound in said network, in order to be highly accessible to the electrolyte and permeable to gas being generated during any overcharge of the cell. Preferably the polymer is polytetrafluoroethylene and the active material is a mixture of nickel and cadmium.
One of the major difficulties with these types of electrodes is their lack of integrity, friability, and the ease with which they are damaged. This results in a large loss of electrode material due to loss of the powdered, active material from the substrate during normal handling and manufacturing operations. Consequently, a significant amount of research and development has been directed towards solving this problem, but without much commercial success. By way of example, U.S. Pat. No. 3,877,986 suggests mixing the powder with about one weight percent of an appropriate binder prior to application of same to a substrate. Any suitable binder is disclosed as being useful such as polyvinyl alcohol, polyvinyl pyrollidone, polymethyl methacrylate, polystyrene and fluorocarbons. U.S. Pat. No. 3,918,989 suggests, as a "shotgun" disclosure, water-soluble resins preferably composed of molecules containing OH or NH groups such as cellulose and substituted cellulose derivative materials, starch and its derivatives, polyvinyl alcohol, pyrollidone, rubber latex resins, ovalbumin, gum arabic, locust bean gum and a significant number of other materials. These water soluble resins are taught as being mixed with a suitable plasticizer which together form a binder material for electrodes composed of active ingredients such as cadmium zinc metal powder, mercuric oxide powder and zinc oxide powder.
U.S. Pat. Nos. 4,476,205 and 4,563,370 disclose using a cured latex rubber or rubber-like material which is applied to the cadmium oxide particles before the cadmium oxide particles are added to the mixture of cadmium and cadmium oxide used to form the pressed cadmium electrode. The resulting electrode product is claimed as being flexible and capable of being rolled up into rolls, but no data is presented with respect to the electrical and pressure characteristics of a rechargeable cell containing said electrode.
There is a need in the art for pressed powder electrodes having improved mechanical strength and integrity which can be handled and processed without falling apart and which, at the same time, do not produce deleterious effects in the electrical properties of rechargeable cells containing same.